Clarification of wet process phosphoric acid



July 15, 1969 w, SATTERWHWE ET AL 3,455,651

CLARIFIGATION OF WET PROCESS PHOSPHORIC ACID Filed April 4, 1966 CENTRIFUGE FIGQ l CENTRIFUGE NA SlF a GYPSUM INVENTORS'.

WILLIAM A SATTERWHITE JACK M. CARTER THOMAS L. CAREY ma dag ATT'Y United States Patent US. Cl. 23-165 Claims ABSTRACT OF THE DISCLOSURE Wet process phosphoric acid containing iron and aluminum phosphates, fluoride complexes and other impurities, is clarified by concentrating the acid by evaporation of Water and the concentrated acid is cooled and aged for growing crystals first at about l85-150 F. for about 8-16 hours, and then at about 120-80 F. for about 8-16 hours, and the mass is then subjected to crystal separation processing.

This invention relates to clarification of wet process phosphoric acid, and more particularly to the production of concentrated clarified wet process phosphoric acid.

Wet process phosphoric acid contains as impurities gypsum (calcium sulfate), gypsum anhydrite, iron and aluminum complex phosphates, and fluoride complexes or compounds. In the concentration of Wet process phosphoric acid, such impurities produce sludge, which not only precipitates during manufacturing stages but also at later stages during storage of the acid.

The starting wet process phosphoric feed acid is usually referred to as 30 percent P 0 acid even though the actual P 0 content varies substantially below and above this stated percentage. Similarly, in the trade parlance, the concentrated acid is referred to as 54 percent P 0 acid although the actual P 0 content may vary as widely as 40-60 percent P 0 acid on a total composition basis.

In the clarification of the wet process phosphoric acid, the feed acid may be first concentrated and thereafter cooled to form crystals, and the acid may then 'be usbjected to crystal separation processing as by centrifuging, filtering, etc. Such processing, however, does not successfully clarify the wet process acid, and not only are there sludge impurities present during processing but there is a substantial post precipitation of soilds during storage of the acid. This remains true even though the cooled acid is held for a period of time to bring about crystal formation. For example, after storage of 54 percent P 0 acid produced by four different companies, it was found that the sludge accumulation after 14 days storage varied from 7 to percent.

We have discovered that the aging and cooling conditions are of great importance, not only for reducing the dissolved impurities down to their normal solubilities at ambient temperature so that post precipitation cannot occur, but also to effect crystal growth on existing crystals so that the particles are sufliciently large that removal can be achieved effectively by centrifugal separation. By aging the acid at an elevated temperature, say, about l85150 F., for a substantial period, say, about 8l6 hours, and then cooling and aging the acid for another about 8l6 hours at about 120-80" F., a substantial improvement in crystal growth is obtained and there is a minimum of post precipitation during the storage of the product.

The foregoing process is enhanced by seeding the feed acid with iron phosphate, this being accomplished preferably through the recycling of sludge from the centrifuge to the feed acid and preferably to an extent that the concentrated acid has an iron phosphate content of about 3.4-6.0 percent (preferably 3.6-4.2%) by weight. The process is further enhanced by reducing the fluoride concentration of the feed acid to be concentrated to 1.5 percent or less, and the calcium sulfate content to about 0.1-0.8 percent CaO equivalent.

A primary object, therefore of the invention is to provide a process for the clarification of wet process phosphoric acid containing a minium of impurity solids and providing a minimum of solids precipitation during storage. A further object is to provide a process in which concentrated wet process phosphoric acid is aged and cooled under conditions most favorable for the forming and removal of crystal impurities and for the minimizing of post precipitation of solids. Other specific objects and advantages will appear as the specification proceeds.

The invention is shown, in an illustrative embodiment, by the accompanying drawing, in which there is set out a diagrammatic layout of apparatus in which the invention may be practiced.

In the illustration given, 10 designates a mixer tank into which the feed acid, referred to as 30 percent acid, is introduced. Into the mixer tank is introduced a sodium ion, such as is provided by sodium sulfate, sodium carbonate, etc., so as to bring about a precipitation of sodium silicofluoride and calcium sulfate (gypsum). From the mixer tank, the acid is passed through a centrifuge 11 for the removal of sodium silicofluoride and gypsum, and the acid partially free of such impurities is passed to a concentrator or evaporator 12, which is preferably of the vacuum type and in which water is evaporated to produce the type of acid product generally referred to as 54 percent P 0 acid. From the evaporator 12, the acid flows into aging tanks 13 where the acid is held at a temperature preferably of about 185-150? F. for about 8-16 hours. Following this time, the acid is cooled and passed through aging tanks 14 for another period of about 8-16 hours, the temperatuer being held in the range of about 80 F. Optimum holding time for tanks 13 was 12 hours, and optimum holding time for the tanks 14 was 12 hours. The acid is gently stirred in the aging tanks to keep the solids in suspension. It will 'be understood that the temperatures and holding times are interrelated and the beneficial effects described can be obtained by a variation of the temperatures and holding times.

From the aging tanks, the acid is passed to a nozzle centrifuge or to a series of nozzle centrifuges 14, the overflow being drawn off through pipe 16 as clarified product. A portion of the underflow may be withdrawn through pipe 16 and sent to an ammoniation plant 16a for the preparation of ammonium phosphate fertilizer. Another portion of the underflow (sludge) may be recycled through line 17 to the mixer tank 10.

The beneficial effect of the aging stages at the temperature of about 150 F. and 120-80 F. is enhanced by recycling through line 17 sufficient iron phosphate to bring the iron phosphate content of the concentrated acid into the range of 3.4-6.0 percent (preferably 3.6-4.2%) by weight.

The process is further enhanced by removing from the feed acid enough of the fluoride complexes to bring the percent thereof in the acid going into the concentrator below about 1.5 percent and to reduce the calcium sulfate content into the range of 0.1-0.8 percent CaO equivalent. By using a solid bowl centrifuge or other eflicient separating equipment, the foregoing impurities are removed as sodium silicofiuoride and gypsum. By way of example, by adding Na SO to the feed acid, the sodium ion reacts with fluosilicate ions, precipitating sodium fluosilicate. The sulfate ion converts to free sulfuric acid and suppresses solubility of (32150.; via common ion effect. Thus, before evaporation to concentrated acid such as to 54 percent P acid, these impurities are reduced to a level sufliciently low to achieve a clarified product acid.

We claim:

1. In a process for clarifying wet process phosphoric acid containing iron phosphates, fluoride complexes and other impurities, the steps of reacting with such wet process phosphoric acid prior to the concentration thereof If desired, the recycled 54 Percent 2 5 acid y be 5 a compound selected from the group consisting of soadded to the incoming 30 percent P 0 a id to Pr du dium sulfate and sodium carbonate to precipitate sodium a Concentration of 35-36 Percent 2 5 aeid- At this eOIlsilicofiuoride and calcium sulfate, centrifuging the acid eehtIetiOIl, it is found that fluorine removed is most F to remove precipitated material and to bring the fluoride ticable. However, satisfactory removal may be accomcontent of h id below 1.5% by i h and h l- Plished at the 2 5 range in the general neighborhood of cium sulfate into the range of 0.1-0.8 CaO equivalent, 30 percent P 0 heating the acid to concentrate it, aging the acid at ap- While the removal of fluorine y be accomplished proximately a constant temperature within the range of thfeugh t use Of celnll'ohtlds Containing Sodium and 1 about 185-150 F. for about 8-16 hours, and at approxitasslum lons and thrfmgh known means for the removal mately a constant temperature within the range of about Q F and fluondes, Such for ,exanfplet steam and 120-80 F. for about 8-16 hours, and centrifuging the anstnpping We prefix to use sodmm.lons because of acid to separate the acid product from sludge impurities. the effective control 1n removing the desired amounts of Z The process of claim 1 in which the Wet process fluorides, and further the use of such compounds as sohos h ri d d h S h t tals dium sulfate and sodium carbonate bring about the rep p o 0 18 See e W on p o p a 6 crys moval of other undesired impurities as above described. before completmg the coPcentrtfmon' Specific examples illustrative of the invention may be f i clalm 1 m whlch Sald Sludge Set out as follows; purities containing iron phosphate crystals are recycled E l I and mixed With the wet process phosphoric acid to be Wet process phosphoric acid containing iron phosconcentrated phate in the range of 3.4412 percent and containing a The proces sof clalm 1 in which the oncentrated CaO equivalent concentration in the range of 0.1-0.6 aged for about 12 hours at 185-150 cooled percent and a fluoride content of less than 1.5 percent, and then aged for about 12 9 about 1204300 was subjected to different aging or cooling conditions as The PmeeSS 0f Chum 3 In Which the recycled Sludge set out in the following table and with the results indibrings the iron Phosphate of the concentrated acid to cated, the test storage period being two weeks: about 3.46.0%.

TABLE Dissolved impurities Resulting Clarified PostPrecip. Total solids F8203, 0110, F, acid solids, acid solids, solids, after storage, Example Aging Cooling percent percent percent percent percent percent percent 12 hrs. at 165 None 1. s5 0. 20 0. 97 7. 23 0.16 a. 0 3. 2 12 hrs. at 165 6 hrs. at 100 1. 7s 0. 06 0. s3 7. 54 0. 37 2. 5 2. 9 12 hrs at 165 1 12 hrs. at; 100 1. s1 0. 07 0.80 9. 00 0. 50 0. 4 0. 9 13 hrs. at 165 None 1. 88 0.10 1. 06 7. as 0. so 3. 0 3. 6 Case 5 18 hrs. at 165 6 hrs. at; 100 1. 76 0.07 0. 01 9.62, 0.16 1. 4 1. 6

As shown by the above table, the conditions in case 3 6. The process of claim 1 in whic hthe compound is produced a stable, clarified 54 percent P 0 acid. Holdsodium sulfate. ing periods of 8-12 hours at each of the temperatures, 7. A process for clarifying wet process phosphoric acid namely, about 165 F. and 100 F., gave total solids of containing iron and aluminum phosphates, fluoride comless than one percent in the product and with a post preplexes, and other impurities, in which process feed acid cipitation of solids of less than one percent. The total is concentrated by evaporation of water and the consolids after two weeks storage was slightly in excess of centrated acid cooled to form crystals and then subjected one percent. to crystal separation processing, characterized in that said acid in the cooling stage is aged for growing crystals first Example H at approximately a constant temperature within the rarige 55 of about 185-l50 F. for from about 8-16 hours and then at approximately a constant temperature within the The process was carried out as described in Example I Tange f about 1 0 f about g houm eXCePt that the aging P at and at 8. In a process for clarifying wet process phosphoric Was 10 hours, the 111 the acid Pf being acid containing iron phosphates, fluoride complexes and percent, and the solids in the product acid after two weeks other impurities the Steps f treating Such wet Process Storage helng Pe phosphoric feed acid prior to concentration with an agent Improved result? 111 the relhoval of cflfstalhzed P selected from the group consisting of sodium sulfate and tlee eeeemphshed y using the Optimum eonfhtlohs sodium carbonate to precipitate fluoride complexes and 0f eglhg after lzohoufs at followed P3 aglng for calcium sulfate, centrifuging the acid to remove fluoride 12 hours at 100 's even thehgh the flllondes and the complexes and calcium sulfate, seeding the feed acid with calcium sulfate were not reduced in content and the feed iron phosphate, heating the acid to concentrate it aging acld was not seeded Wlth PhCPSPhateZThe agmg steps the acid at approximately a constant temperature within however, were greatly enhanced 1n effectiveness when the the fame of about 185 1 500 F for about hours feed acid was seeded with iron phosphate and the fluoa in t t 1 tq t t t ride complexes and calcium sulfate reduced in amount. 6 am a approxlma e y a E empera um While in the foregoing specification we have set out a the range, about 12 0 8O for about 846 specific process in considerable detail for the purpose of hours and Cenmfugmg t aclFL illustrating embodiments of our invention, it will be un- The Process of 6131111 8 111 Whlch a Portlon of the derstood that such details may be varied Widely by those centrifuged acid containing hon PhOSPhate is l'eeycled skilled in the art without departing from the spirit of our and miXed With the feed aeid to bring the hon Phosphate invention.

content of the concentrated acid to about 3.6-4.2 percent.

5 10. The process of claim 8 in which the fluoride complexes are removed to a point below 1.5 percent by weight.

6 References Cited UNITED STATES PATENTS 2,883,266 4/1959 Hodges et a1. 23--I65 2,917,367 12/1959 Hodges et a1 23165 2,936,888 5/1960 Williams 23-165 X 6 9/1965 Crawford et a1 23l65 8/1967 Mazurek et al 23-305 US. Cl. X.R. 

